1. Field of the Invention
The application relates to a method utilized in a wireless communication system and a communication device thereof, and more particularly, to a method of performing power headroom reporting in a wireless communication system and a related communication device.
2. Description of the Prior Art
Long Term Evolution wireless communication system (LTE system), an advanced high-speed wireless communication system established upon the 3G mobile telecommunication system, supports only packet-switched transmission, and tends to implement both Medium Access Control (MAC) layer and Radio Link Control (RLC) layer in one single communication site, such as in Node B (NB) alone rather than in NB and RNC (Radio Network Controller) respectively, so that the system structure becomes simple.
The power headroom report is generated by a power headroom reporting (PHR) procedure, and is used to provide the serving eNB with information about the difference between the maximum UE transmission (TX) power and an estimated TX power for Uplink Share Channel (UL-SCH). With power headroom (PH) information sent by the UE, the network can allocate radio resource to the UE and make schedule decision more efficiently. In the PHR procedure, the UE uses a MAC control element in a MAC protocol data unit (PDU) to carry PH information.
Toward advanced high-speed wireless communication system, such as transmitting data in a higher peak data rate, LTE-Advanced system is standardized by the 3rd Generation Partnership Project (3GPP) as an enhancement of LTE system. LTE-Advanced system targets faster switching between power states, improves performance at the cell edge, and includes subjects, such as bandwidth extension, coordinated multipoint transmission/reception (COMP), uplink multiple input multiple output (MIMO), etc.
For bandwidth extension, carrier aggregation (CA) is introduced to the LTE-Advanced system for extension to wider bandwidth, where two or more component carriers are aggregated, for supporting wider transmission bandwidths (for example up to 100 MHz) and for spectrum aggregation. According to carrier aggregation capability, multiple component carriers are aggregated into overall wider bandwidth, where the UE can establish multiple links corresponding to the multiple (downlink and uplink) component carriers for simultaneously receiving and transmitting.
In carrier aggregation, the UE only has one RRC connection with the network. At RRC connection establishment/re-establishment/handover, one serving cell provides the NAS mobility information, and at RRC connection re-establishment/handover, one serving cell provides the security input. This cell is referred to as a Primary serving cell (PCell). In the downlink, the carrier corresponding to the PCell is the Downlink Primary Component Carrier (DL PCC) while in the uplink it is the Uplink Primary Component Carrier (UL PCC). In addition, cells other than the PCell are named secondary serving cell (SCell).
It is possible to configure a UE to aggregate a different number of CCs originating from the same eNB and of possibly different bandwidths in the UL and the DL. Note that, the number of DL CCs that can be configured depends on the DL aggregation capability of the UE, the number of UL CCs that can be configured depends on the UL aggregation capability of the UE, it is not possible to configure a UE with more UL CCs than DL CCs, and in typical TDD deployments, the number of CCs and the bandwidth of each CC in UL and DL is the same. In addition, the PCell is always configured with DL PCC and UL PCC, and the SCell can be configured only with DL SCC.
In LTE-A system (e.g. Rel-10), parallel PUCCH and PUSCH transmission have been supported. Consequently, eNB should take into account PUCCH and PUSCH transmission power contributing to the UE current transmission power, so as to assist eNB to schedule PUSCH and PUCCH. Note that, for uplink resource, the PCell has PUSCH and PUCCH, but SCell only has PUSCH.
For a UE supporting a single uplink component carrier in the LTE system, the UE reports a PHR only for the one uplink component carrier. For a UE supporting multiple uplink component carriers in the LTE-Advanced system, the UE has to report PHRs for multiple uplink component carriers. However, the network (e.g. eNB) may not well control transmission power for the uplink component carriers even if the network knows the power status of each uplink component carrier of the UE. Based on the TX architecture of the UE, multiple uplink component carriers may share the same power amplifier (PA). For example, a UE configured with one PA for TX is configured with two uplink component carriers. When a PHR is triggered, the UE reports PH information respectively for the component carriers to the network based on the concept of the LTE-Advanced and/or LTE system. Since the power resource of PA is shared by the two component carriers, if the UE allocates power resource for the first component carrier according to the PH information of the first component carrier without consideration of the PH information of the second component carrier, and allocates power resource for the second component carrier according to the PH information of the second component carrier without consideration of the PH information of the first component carrier, the sum of the allocated power resources for the first component carrier and the second component carrier may exceed power headroom of the PA. Thus, the UE shall not allocate power resource according to PH information of each component carrier due to PA power limitation.